September 07, 2011

Large Diurnal Temperature Range

An interesting aspect of the current weather regime is the large diurnal (daily) temperature range--the difference in temperature between the daily highs and lows.  At a number of Northwest locations we have seen highs in the 80s and 90s, while temperature have plummeted at night into the lower 50s and even some 40s, with some dropping into the 30s.   I am talking about surface air temperatures here, measured at roughly 2 meters. Some good examples:

Baker, Oregon--high of 90 and a low of 39F:  a diurnal range of 51F!
Olympia,WA--high of 85 and low of 44:  range of 41F

Or consider the Turnbull climate reference network site near Spokane that had  a 51 deg temperature range today with a low of  36 and a high of 87 on Tuesday.  What is really amazing is the range of surface (ground) temperature that on Tuesday jumped from 32 F to 114 F (82F!!) on Tuesday.  You read that right....from frost on the ground to 114F in one day.   That will crack some rocks!

 Here are graphs of temperature for the last two weeks--all of which show the huge range.  First, Sea-Tac, then Pasco, and finally Spokane.  All have a much greater range than normal.
So why such a big range?  Hint:  we often observe such big daily swings during late summer and early fall.

We start with fairly warm aloft and the sun being still fairly strong....that allows warming.
We have weak offshore flow aloft...that keeps the low clouds and marine influence at bay.
We have clear or nearly clear skies...that allows good infrared radiational cooling to space--and thus good temperature falls at night.
And nights are getting longer--that gives more time for nighttime cooling.  And the relatively equal time for heating and cooling at this time of the year is helpful

Put this all together and you get one big temperature range.  During the winter the range is far less in general, particularly because the cloud cover reduces heating during the day and infrared cooling at night.

Now for the controversial part of this blog.   There seems to be some difference in opinion whether a large temperature range is good for viniculture.   Does a big range help or hurt the quality of grapes used in wine-making?  Some online sources claim that such a range is good since it has the effect of producing high acid and high sugar content as the grapes' exposure to sunlight increases the ripening qualities while the sudden drop in temperature at night preserves the balance of natural acids in the grape.  Others, like the book by Gladstone, claims that a narrow temperature range is good.  Any wine experts read this blog?  What is the correct story?  I have always found the meteorology of wine making fascinating...and will climate change make eastern Washington wines even better?  Perhaps in another blog.


  1. Cliff, a question about the forest fire in the Olympic Mts. From my perspective, 48th & Phinney, it looks like the smoke is drifting northwards out over the strait and north sound. But the prevailing winds are out of the N/NW. What could account for this?


  2. Cliff, a question about the fire in the Olympic Mts. From our perspective at 50th and Phinney, it looks like the smoke is drifting northwards. Yet the prevailing winds are N/NW, right? What could account for this? Is there a close-up satellite view I should look at?

  3. Great question about diurnal temperatures and wine-grape growing, Cliff. It is certainly widely said in the West Coast wine industry that large diurnal temperature variation is a good thing, as you describe. But I've wondered myself if we tend to hear that line because the areas warm and sunny enough to ripen grapes (but also not too hot and dry) happen to have pretty big diurnal temperature swings. Since it's a fact of our vineyards, naturally we include it as a plus in our marketing (the side of the business I am in). But is it scientifically verified? I have not seen that. I do know that Napa wines are more expensive and generally more highly regarded than, say, Paso Robles wines -- and Napa has relatively smaller diurnal variation than Paso, where it can by 105 at 3 p.m. and 48 at 5:30 a.m. But I don't think that proves anything. I've not read the John Gladstones book ("Wine, Terroir and Climate Change" for those looking for it), but found an excellent summary here that others might find illuminating:

  4. The wine question:

    There are a lot of good wines composed of grapes that aren't exposed to a "large diurnal temperature range."

    The real question is what wines embody those qualities mentioned in your post: that is, which wines have the richest flavours, are more acetic and have a slightly lower alcohol content? Many Italian wines fit this description AND these are the very qualities that make Italian wines some of the best to pair with food.

  5. Hello Cliff,
    I am a vintner and grape grower with over 30 years experience in France, California and Washington. I have found with that the cool nights do indeed preserve great acidity in our WA wines; it is common to have to add back acid in CA and infrequent here. (few cool nights down there during harvest...) I am scientifically trained (Univ Calif Davis- viticulture and enology), though this belief is only based on my experience in winemaking and tasting!

    Brent Charnley, Lopez Island Vineyards and Winery

  6. Hello Cliff,
    I am a long time vintner and grape grower, with over 30 years experience in France, California and Washington. I beleive that the cool nights of our region and the time of year we harvest, do indeed help preserve the acidity in our wines here in WA. In CA it is very common to have to add back acid to the wine, while that is infrequent here. While this is not scientifically verified on my part, I do have a science training in enology and viticulture from Univ Calif Davis, so I feel this opinion is coming from proper persepective, based on my own lab work, winemaking, and of course, tasting!
    Cheers, Brent Charnley, Lopez Island Vineyard and Winery

    PS- Spelling notes to comments:
    viticulture not viniculture
    acidic not acedic (vinegar)

  7. Having lived in the Central Valley of California for 15+ yrs, these are totally normal swings in temps. We enjoyed the cool evenings, and so did all the grapes. San Joaquin and Stanislaus Counties are home to the 2 largest wineries in the world and grow million of $ of wine.

  8. All I know is that I love the temperature swings -- the reason I (usually) start to dislike the 85+ range is that it just doesn't cool off early enough and sleeping is miserable. With these temperature swings and earlier sunsets, my top-floor bedroom is downright pleasant at night.

  9. Dear sir This is a very helpfull post . thank you

  10. Cliff,

    It all depends on several factors. High swings in the diurnal can be beneficial and also a problem depending on how hot it gets. In Eastern WA there is a huge change in temperatures from day to night compared to most growing regions and that is a blessing because of how hot it gets and how long the summer is. Growing grapes on the east coast is a different story because they have little diurnal changes and as we know most east coast wines don't match the quality from Europe and the West Coast. Small shifts in diurnal means that acids will come out of the grape more quickly if the high temperatures stay high. It's all about heat accumulation. In Gladstones book, "Viticulture and Enviroment" he maps out growing regions around the world based on Growing Degree Days (with modifications). The GDD model is a pretty good way to find out what will ripen in your backyard... Bordeaux isn't a very hot place but they grow the same grapes as Napa. The diurnal shift in Bordeaux is much smaller than Napa which can be huge. High daytime temperatures also make the vines shut down, so anything over 90 degrees or so and the vines just don't do anything. So cooler temperatures with a smaller shift is more efficient than large shift, but a more complex wine is created by hanging around longer on the vine... it's so complicated!

    Steve Snyder
    Owner, Hollywood Hill Vineyards
    Viticulture Instructor South Seattle Community College

  11. Here are some comments from my brother-in-law, Dr. Wade Wolfe, Washington State's foremost viticulturist and (with my sister Becky Yeaman) owner of Thurston Wolfe winery

    "A few comments on temperatures and their affect on viticulture. Inland (continental) areas tend to have higher summertime diuranal fluctuations, especially at higher latitudes because of lack of cloud cover and longer day lengths. It is not uncommon for us to have 50 degree daily swings. I have not thought about them being greatest in the late summer, early fall, but it is consistent with the shortening daylengths combined with still intense solar radiation. I would say your blogger is right on with regards to the affects on sugar and acid. Despite the daily high temperatures, the mean temperatures are still lower in the fall when fruit is ripening compared to California (lower latitudes). At that time, the vine is degrading malic acid and the rate is directly proportionate to temperature. At the same time herbal flavors are declining and fruit flavors are increasing. Whether this translates into improved grape and wine quality depends somewhat on where the varieties originate. The northern European varieties tend to do best with lower diurnal fluctuations and overall heat units, while the southern European varieties tend to do better with greater fluctuation and heat units. There appears to be a few exceptions to this rule, one being Chardonnay and another Riesling, at least to some degree. But this is why Pinot noir is usually considered to do better in maritime-like conditions rather than continental.

    Regarding the hottest location in the Northwest, locally the Hanford site usually registers the highest temperatures for the reasons that the blogger cites, though it is not much warmer than the neighboring Tri-Cities. As far as record high temperatures, Boise has that claim to fame with 120 degrees. I think the hottest I have experienced here is 113."

  12. Phinney Four asked about the smoke--the low level wind is northerly, the upper level wind southerly. You can see that in the Sandpoint profiler

    That is still true today when you look at the profiler--it may not be if you click on that link in a few days. It also depends at what altitude the smoke settles in at--probably a lot of smoke in the nighttime hours is staying at low altitudes, and gets blown south. If the fire is active, it can pump smoke up higher, just like a cumulus cloud building up. That smoke moves north, in the southerly wind aloft. Good observation Phinney Four.

  13. While I don't have as much winemaking or viticulture experience as Brent or Steve, I can comment on my observations the past few years.

    In 2009 (a hot year with large diurnal change as I recall) the grapes I used from Red Mountain near Benton City had ample sugars and less acid. I had to add acid that year during fermentation. In 2010 (a cooler year with less diurnal change) sugars were lower and acid was generally higher. Everything I've learned supports Cliff's opinion, but it could vary based on a lot of factors. Like Steve said, it's complicated.

    Scott Greenberg, Owner/Winemaker
    Convergence Zone Cellars, Woodinville

  14. The answer to me lies in the year. Yes the warm days are good for ripening, if they are not too warm and depending upon the variety of grape you are growing. Yes the cool nights do help in controlling the acid breakdown in the grapes. The big question is, on a cool year like the one we are having is, will there be too much acid in the wine at harvest. This may be a problem the wine maker will have to deal with. On a normal year, if a grower has properly selected their grapes to their local environment the warm days and cool nights are good for wine quality. I question that this year may be a little out of balance; too cool for many reds to do their best. The final outcome will depend upon the weather for the remainder of the growing season.
    Ed the Grape Guy

  15. Very interesting and helpful blog. Thanks.

  16. Lovely photo. Is that the Wahluke slope?

  17. Cliff, a question. Could you write about the pollen? I've been snifflin' and coughin' up a storm this summer, and so have a bunch of people I know.

    Went over to Eastern Oregon a while back and it all cleared up. Get back to Seattle, and bingo, it hits. I've lived here since the mid'-90s and am not especially allergy prone. And it's not just me.

    What's up?

  18. Where in the Northwest was this low of 32 followed by a high of 114?

    I've reread this several times and don't see the location.


  19. A small cloud came over Montlake today and sprinkled a little rain at 8:30am. Moderate sized raindrops left little wet spots on the pavement for about half an hour before they dried. Otherwise a very blue sky. Weather report was "mostly clear" during this period.

  20. Hi Cliff - The most important effect of our relatively large diurnal temperature variations is to slow down the accumulation of sugars in the ripening grapes and decrease the rate at which acidity is lost, while allowing flavor and aroma compounds, which seem to need TIME as much or more than heat, to develop. This produces "balance" in the grapes between these components. The diurnal variations are especially pronounced late August - mid September when days are getting shorter but it's still "summer". Cold-air pooling in valleys with restricted air drainage (e.g. Walla Walla and Yakima Valleys), amplifies the effect at lower elevations.

  21. Anyone know how to contact Cliff Mass for a request? The news says there is evidence of a building La Nina occurence for this winter. I would like to ask him to address La Nina and its usual effects on our weather for the past winters, and probable coming winter weather

  22. There was a forest fire next to UBC yesterday. Great pictures. A symptom of the dry late summer.

  23. John @ KBCS, I've corresponded with Cliff several times at his school email address: Cliff's email

  24. Dr. Mass, follow up on John's comment, I hope you address the re-appearance of the La Nina. Scott Sistek has already written about it.

    A UK forecasting group -which forecast last year's snowy December in the UK- has done a forecast for the US this winter.

    It's calling for a severe winter in our region.

  25. Small temperature changes during the growing phase.
    Bigger drop before harvest to "finish" them off.

  26. You could add a few more reasons for the diurnal contrast (which is different from sheer temperature swing):

    1. Low humidity which stays low because there's no soil moisture to feed it. Makes air easier to heat and cool.

    2. Little wind to offer relief from the hot sun.

    3. Lower sun angle means the sunlight hits us more from the side, and that makes our faces and vertical body surfaces feel especially hot.

    4. We're not used to cool temperatures this time of year, and that makes the late nights and mornings feel especially chilly.


    Late February and March can also be good times for contrasty weather if the skies are clear, because:

    1. The sun is getting quite strong and that contributes to daytime heating;

    2. The strong sun on our faces makes things feel warmer still;

    3. We're used to the cold so that temps of 55-65 feel quite warm;

    4. The clear nights are still plenty long to get cold, and it's usually colder after midnight than on a more typical rainy/cloudy day. That makes the late nights and mornings feel sharp since we're used to slightly milder nights.

    5. Offshore flow can begin to get a downsloping component that it usually doesn't have in the middle of winter, contributing to a bit of compressional heating.



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